In the past decade research in cellular immunology has established that thymus (T) and bone marrow (B) derived lymphocytes collaborate in the immune response to a variety of antigens in the production of humoral antibody by B cells. T cells effect cell mediated immune reactions. These conclusions were based on research involving extensive in vivo and in vitro experiments using animals; mostly syngeneic. The applicability of these conclusions to the study of the immune response in man is not necessarily possible. Since the in vivo studies of the immune response in man are not practical, in vitro approaches must be developed. To date there is no in vitro system to study the primary immune response of human lymphocytes to antigen or to study cellular collaboration. We present a model, based on combinations of mitogens, to study and extend our observation that cellular collaboration can be induced in the blastogenic response of human lymphocytes. We have used combinations of phytohemagglutinin (PHA) or Concanavalin A (Con A) and LPS to induced synergism in the blastogenic response, as measured by increased DNA synthesis, in human peripheral blood lymphocytes (PBL). Our system includes a micro culture method. We propose to: 1. study the mechanism of action of combinations of PHA or Con A and LPS on the synergistic blastogenic response of human PBL: 2. study and extend our basic model of synergism in the blastogenic response of human PBL: 3. continue our preliminary observations on the role(s) of purified human T an B cells and monocytes as they function in the synergistic blastogenic response of human PBL to combinations of PHA or Con A and LPS: 4. determine if our model system can be used to detect abnormalities in various human disease states.